The invention relates to an exposure device having at least one exposure arrangement for the structured exposure of a surface, for example the surface of a wafer, as is known generically from U.S. Pat. No. 6,876,494 B2.
In conventional exposure arrangements, as are used in optical lithography, a wafer is structured, that is to say exposed using a predetermined structure pattern (exposure structure), in which a mask is projected onto a wafer or is brought into contact therewith. Disadvantages here are the manufacturing complexity in producing the masks, which is significant in particular during flat-panel display production owing to the necessary size, and the limited possible use in each case only for exposure with a single exposure structure.
U.S. Pat. No. 6,876,494 B2 discloses an exposure arrangement with which light in a predetermined exposure structure (structure pattern) is projected onto a wafer by spatially modulating the light via a micromirror array.
Like other exposure arrangements known from the prior art, the exposure arrangement disclosed here includes, arranged along an optical axis, a planar emitter, a micromirror array (referred to below as DMD=Digital Micro Mirror Device), a condenser system, which is arranged upstream of the DMD in the radiation direction and homogenizes the ray bundle emitted by the planar emitter, for illuminating the DMD, a microlens array (referred to below as MLA) having a number and arrangement of microlenses corresponding to the micromirrors of the DMD, a first imaging system, which is arranged upstream of the MLA in the radiation direction, for imaging the portions of the ray bundle that are reflected by the micromirrors onto the respectively corresponding microlenses, and a second optically imaging system, which images the light portions focused by the microlenses onto the surface to be exposed.
It is the object of U.S. Pat. No. 6,876,494 B2 to increase the quality of the imaging by way of two different measures.
For this purpose, firstly an exposure arrangement is intended to be provided, in which a magnifying imaging system is produced by combining optically imaging systems with a microlens array, wherein a deterioration of the absorption ratio, caused by distortion, is intended to be avoided.
Secondly, an apparatus is intended to be provided, in which a microlens array is combined with an aperture array, and the relative spacing between the microlens array and the aperture array in the direction of the optical axis can be set precisely.
The first object is achieved by configuring the first and the second optically imaging system as image-magnifying systems and by the desired magnification resulting as the product of the magnification of both systems. It is therefore possible to configure the two individual imaging systems such that they are small, as a result of which a deterioration of the distortion characteristic is avoided and a satisfactory absorption ratio is achieved.
The second object is achieved by forming, on the microlens array or on the aperture array, at least one protrusion in the direction of the other array, which protrusion has a predetermined height in the direction of the optical axis and determines the spacing of the arrays. The relative position of the arrays with respect to each other in the direction of the optical axis can thus be set precisely, with the result that the apertures are situated in the focal planes of the microlenses.
Instead of the at least one protrusion, it is also possible for a spacer with a predetermined thickness to be placed between the microlens array and the aperture array.
In summary, the measures achieving the object in U.S. Pat. No. 6,876,494 B2 serve to improve the quality of the structured exposure by virtue of the structure being imaged more sharply and with a more homogeneous intensity.
Considerations relating to an increase in throughput rate can also be found in U.S. Pat. No. 6,876,494 B2.
The throughput rate is the rate at which a surface to be exposed is completely exposed. This rate is determined by the minimum time interval with which the exposure can take place and by the size of the partial surface, as a ratio of the total surface to be exposed, that is exposed in each case at the same time.
This ratio can be improved if, rather than just one exposure arrangement, a plurality of exposure arrangements, which in each case expose a partial surface of the total surface to be exposed with an exposure structure and together form an exposure device, are arranged next to one another, for example, in a row or in a plurality of rows which are offset relative to one another.
The minimum time interval between exposures, which determines the maximum exposure frequency, is limited by the loading time for a structure pattern into the memory cells of the DMD (each micromirror has an associated memory cell). This loading time results from the number of duty cycles for processing the command, the clock frequency, which is currently 400 MHz, and the number of memory cells to be loaded at the same time (referred to as blocks).
The modulation rate is the number of possible modulations of a DMD, or of the mirror elements of the DMD, which play a role in the formation of the structure pattern for the exposure, per unit time.
For a switching operation of a DMD with, for example, 1024×768 mirror elements to be switched, as is available from Texas Instruments under the name: DLP Discovery.? XGA 2xLVDS 12° Type A, and a DMD clock frequency of 400 MHz, the result is a typical modulation frequency of 20 kfps (kilo frames per second).
U.S. Pat. No. 6,876,494 B2 proposes to increase the modulation rate and thus the maximum exposure frequency by using only some of the mirror elements of a DMD to form the exposure structure. In this case, all the mirror elements over the width of the DMD which are arranged perpendicular to the transport direction of the surface to be exposed should be used, but only some of the mirror elements that are arranged in the transport direction.
This solution has several side effects:
In the case of an already existing exposure arrangement, the DMD is illuminated over a given surface area. However, if only the x-th part of the DMD surface is used, the (1-x)th part of the illuminated surface remains unused (loss of efficiency).
If the exposure arrangement is variable or already designed for a smaller surface, the following side effects exist:
A defined rotation of the DMD (U.S. Pat. No. 6,876,494 B2; cf. FIG. 8A and 8B “p2”), which is intended to increase the resolution (FIG. 8B “p2”), results in a specific number of repetitions of the exposures of a given point. The repeated exposure of a point improves the uniformity of the exposure. However, if fewer elements are used, the number of repetitions drops and the dose per repetition needs to be increased. The uniformity, which results from averaging, decreases.